1. Field of the Invention
The present invention relates to a gas generator for an air bag for protecting a vehicle occupant from an impact of vehicle crash and so on.
2. Description of the Related Art
An air bag system installed in a vehicle, upon receiving an impact such as that due to a crash, develops an inflatable bag and prevents a situation in which a vehicle occupant collides with components in a vehicle (a steering wheel, a windshield, et al.), to protect the vehicle occupant. An air bag is inflated using an inflation gas, and a gas generator is used to generate the gas. Since the time between receiving an impact and developing a bag is very short, an ability of generating a sufficient gas to rapidly develop the air bag in a short period of time is required to such a gas generator.
However, on the other hand, when developing speed is increased uniformly to develop rapidly an air bag in a short period of time, there may occurred a case in which an impact of a rapidly inflating bag affects a vehicle occupant. For example, in comparing a vehicle's crash at low speed with a vehicle's crash at high speed, the affect of an impact to a vehicle occupant are different, and as is obvious, the former has less impact to a vehicle occupant. So, if the bag is developed rapidly even in a crash at low speed just like a crash at high speed, the development of a bag itself possibly affect the vehicle occupant.
A gas generator generating gas in multi-stages is proposed in order to prevent this.
For example, in JP-A 2001-97175, a gas generator in which two combustion chambers are disposed, a gas generating agent in each combustion chamber burns independently, and the generated gas amount is adjusted according to a degree of an impact, is proposed.
In addition, there is JP-A 2001-239914 as a related art to the present application. In JP-A 2001-239914, an inflator for an air bag in which a pipe 9 and a pipe 39 are arranged in a housing, a booster propellant 33 and an initiator 23 igniting it are disposed in the pipe 39, and a propellant 12 and an initiator 13 are disposed in another pipe 9, is shown. In addition, a main propellant 31 is disposed outside the pipes 9 and 39 (inside of the housing: a first combustion chamber 43). Furthermore, in the pipes 9 and 39, gas ejecting ports 9e and 39e are formed respectively, and they communicate with the first combustion chamber.
However, as described that “the second combustion chamber is ignited 100 msec after the detection of a crash, for example, in a light crash. This is because, in a light crash, a low inflating pressure of an air bag is enough (paragraph number [0024])”, in the gas generator in this document, when the crash impact is small, a propellant 12 in the second combustion chamber does not participate in an inflation of a bag in a crash substantially. In addition, since the gas ejecting port 9e in a cylindrical side plate with large diameter 9c is choked with a valve plate 11 but the gas ejecting port 39e in a cylindrical side plate 39c is not choked with a valve plate 11, in the case of burning the propellant 12 in the second combustion chamber, a booster propellant 33 is also burnt, that is all the propellant is burnt. So, in the inflator for an air bag of JP-A 2001-239914, there is a case in which a power of a bag development becomes large even in a light crash, and a developing pressure of a bag cannot be adjusted depending on a degree of the impact.
In addition, in US-A 2002/0050703, a gas generator having three chambers in a housing is disclosed. In a main combustion chamber, a first combustion chamber 42 and a second combustion chamber 44, a main propellant charge 82, a first ignition compound 78, and a second ignition compound 80 are disposed respectively, and the first ignition compound 78 and the second ignition compound 80 are ignited by an igniter 48 and an igniter 58, respectively. In addition, the first combustion chamber 42 and the main combustion chamber communicate with each other via a communicating port 50, and the second combustion chamber 44 and the main combustion chamber communicate with each other via a communicating port 60.
However, since the communicating port 50 connecting the first combustion chamber 42 and the main combustion chamber is only closed with a first aluminum piece from the first combustion chamber side, and a shield 84 as is disposed on the communicating port 60 is not arranged, when the propellant 80 in the second combustion chamber 44 is burnt, the first ignition compound 78 is also burnt, that is all the ignition compound is burnt. So, just like the case of JP-A 2001-239914, there is a case in which the gas generator shown in US-A 2002/0050703 possibly has a large power for developing a bag in a light crash, and a developing pressure of the bag cannot be adjusted depending on a degree of the impact.